The present invention relates to the manufacture of plastic sheets of good optical quality, and especially to improvements in methods and apparatus for making such sheets by depositing a liquid mass upon a moving substrate to form a liquid layer, causing said layer to solidify on said substrate to form a plastic sheet, and thereafter removing said sheet from the substrate.
The terms "plastic sheet" or "sheet" are used herein to include a thin film made up of one or more layers of a resinous or polymeric material, as well as a thicker film or plate formed from one or more such layers.
There are a variety of applications in which it is desired to produce a sheet of resinous or polymeric material of good optical quality, i.e. one which is transparent PG,5 and homogeneous in its optical properties over its full useful area. In general, this means that it should be as free as possible of optical defects throughout its thickness and at its opposite surfaces, and that such opposite surfaces should be of precise predetermined configurations, usually flat and parallel to each other. One application with respect to which the invention will be described with particularity is in the manufacture of anti-lacerative, self-healing plastic sheets useful on glazings of various types.
An important example of such use arises in connection with the glass windows of motor vehicles--especially the windshields thereof--where severe lacerations of the face or other parts of the body of a driver or passenger by sharp particles of glass frequently result when a window is shattered in an accident. Such problems can be obviated by use of an anti-lacerative plastic sheet adhered to the inner side of the glass window, which sheet serves to shield and protect passenger and driver from the sharp edges of the broken glass. An improved safety windshield incorporating an anti-lacerative, self-healing plastic sheet is described in U.S. Pat. No. 4,232,080 of Roger Orain et al, issued Nov. 4, 1980 and assigned to the same assignee as the present invention.
It is desirable in many cases that such anti-lacerative sheet not be susceptible to permanent marring or scratching in normal use, and in some cases it is desirable that it be energy-absorbent in the sense that it will take up or absorb energy of an impact with the head or other part of a passenger or driver, for example by stretching so as to cushion the shock of such impact. Anti-lacerative sheets having self-healing properties are now known, and such sheets which are energy-absorbent have also been proposed.
An excellent material for use in preparing a self-healing, anti-lacerative sheet is described in U.S. Pat. No. 3,979,548 of Helmer Raedisch et al, issued Sep. 7, 1976 and assigned to the same assignee as the present invention.
To aid in attaching such thermosetting polyurethane sheets to glass, it is known to make the sheet in the form of a two-ply laminate made up of a ply of thermosetting polyurethane having self-healing, anti-lacerative properties, and a ply of a thermoplastic polyurethane bonded to the thermosetting sheet, the thermoplastic ply being non-tacky at room temperature but softening at an elevated temperature to function as an adhesive; when the sheet is heated, its thermoplastic ply pressed against the glass, and the sheet then cooled in situ, the thermoplastic ply forms a good bond with the glass. Such a two-ply sheet is described and claimed in U.S. patent application Ser. No. 70,732 of H. Agethen et al, filed Aug. 29, 1979, now abandoned. An especially advantageous form of the adhesion-providing ply, which includes the use of a a blocked polyurethane, is described and claimed in U.S. patent application Ser. No. 213,225 of Gerard Daude et al, filed Dec. 5, 1980. The disclosures of these applications, each of which is assigned to the same assignee as the present invention, is incorporated herein by reference. Such anti-lacerative, self-healing mono-ply and multi-ply sheets are also suitable for use on the glazings of eye glasses, goggles, windows of buildings, etc.
It is known to manufacture sheets of the above-described type by depositing a liquid material on a polished metal drum of large diameter, or on endless metal strips having polished surfaces, followed by heating to produce evaporation of the solvent(s) and/or to produce a polymerization reaction and a solidification of the plastics material in situ on the substrate. The sheet so-formed is then removed from the substrate for subsequent use.
This process is not without drawbacks, since the quality of the polish of the substrate is adversely affected by its continuous reutilization. Consequently, the optical quality of the sheet diminishes as the substrate wears down due to reuse.
It is also known to manufacture such plastic sheet by casting a liquid mass on a substrate formed by a succession of contiguous sheets of glass, as described in U.S. Pat. No. 4,136,214 of Claude Bourelier et al, issued Jan. 23, 1979, also assigned to the same assignee as the present invention. These sheets of glass are also re-used, and after repeated use, their surface becomes marred by scratches and other surface defects which require their replacement to avoid substantial optical defects in the sheet of plastic material so made. In such a process, the presence of joints between the sheets of glass also creates difficulties. Furthermore, the implementation of the process requires a substantial consumption of energy since it is necessary to heat up the thick glass substrate as it moves through the deposition and treatment zones.
To prevent alteration of the optical quality of the sheet of plastic material through wear or surface damage to the casting substrate, it has been proposed to cast the liquid material on an endless ribbon of glass, which is placed in continuous motion in a horizontal position immediately after the making of the glass ribbon by the float method, for example, as described in U.S. Pat. No. 3,990,876 of Pierre R. Heymes et al issued Nov. 9, 1976, also assigned to the same assignee as the present invention. This ensures the continuous availability of a fresh casting surface free from defects. This process has the drawback that it lacks versatility and economy since the glass-ribbon producing units are of substantial size and expense, and their use can only be envisioned in large-scale production of sheets of plastic material, with large capital and operating costs. In addition, it again requires substantial energy and time to change the temperature of the glass ribbon as required, as it passes through the deposition, solidifying and cooling zones of the sheet-forming apparatus.
Accordingly, it is an object of the invention to provide a new and useful method and apparatus for the manufacture of plastic sheets of good optical quality, and to provide such sheets of consistent good quality at reduced manufacturing expense by a process which requires less energy consumption and processing time than other previously-known processes.